radv: mark const structs as extern in header file to avoid lto damage

radv: mark const structs as extern in header file to avoid lto damage

The copr repo from che was using LTO and he reported radv broke
recently with it. When testing with lto builds here I noticed
that we weren't seeing any instance extensions reported.

It appears LTO was treating the const without extern as an empty
struct, this is possibly a gcc bug, but we can work around it
just by marking these with extern.

Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>